The present invention relates to the pyrolysis of hydrocarbons and particularly to the conversion of paraffins to olefins by a catalytic/oxidative promoted pyrolysis.
The steam cracking (pyrolysis) of hydrocarbons for the production of petrochemicals is almost exclusively carried out in tubular coils located in fired heaters. The pyrolysis section is considered the heart of an olefin plant and has the greatest influence on the economics of the overall plant.
The hydrocarbon feedstock may be any one of the wide variety of typical cracking feedstocks such as methane, ethane, propane, butane, mixtures of these gases, naphthas, gas oils, etc. The product stream contains a variety of components the concentration of which are dependent in part upon the feed selected. In the conventional pyrolysis process, vaporized feedstock is fed together with dilution steam to a tubular reactor located within a fired heater. The quantity of dilution steam required is dependent upon the feedstock selected; lighter feedstocks such as ethane require lower steam (0.2 lb./lb. feed), while heavier feedstocks such as naphtha and gas oils require steam/feed ratios of 0.5 to 1.0. The dilution steam has the dual function of lowering the hydrocarbon pressure and reducing the carburization rate of the pyrolysis coils.
In a typical pyrolysis process, the steam/feed mixture is preheated to a temperature just below the onset of the cracking reaction, typically 650.degree. C. This preheat occurs in the convection section of the ethylene heater. The mix then passes to the radiant section where the pyrolysis reactions occur. Generally the residence time in the pyrolysis coil is in the range of 0.2 to 0.4 seconds and outlet temperatures for the reaction are on the order of 700.degree. to 900.degree. C. The reactions that result in the transformation of saturated hydrocarbons to olefins are highly endothermic thus requiring high levels of heat input. This heat input must occur at the elevated reaction temperatures. It is generally recognized in the industry that for most feedstocks, and especially for heavier feedstocks such as naphtha, shorter residence times will lead to higher selectivity to ethylene and propylene since secondary degradation reactions will be reduced. Further it is recognized that the lower the partial pressure of the hydrocarbon within the reaction environment, the higher the selectivity.
The flue gas temperatures in the radiant section of the fired heater are typically above 1,100.degree. C. In a conventional design, approximately 32 to 40% of the heat fired as fuel into the heater is transferred into the coils in the radiant section. The balance of the heat is recovered in the convection section either as feed preheat or as steam generation. Given the limitation of small tube volume to achieve short residence times and the high temperatures of the process, heat transfer into the reaction tube is difficult. High heat fluxes (Q/A) are used and the operating tube metal temperatures are close to the mechanical limits for even exotic metallurgies. In most cases, tube metal temperatures limit the extent to which residence time can be reduced as a result of a combination of higher process temperatures required at the coil outlet and the reduced tube length (hence tube surface area) which results in higher flux and thus higher tube metal temperatures.
The product gas is sent to a closely connected transfer line exchanger where the temperature is rapidly reduced. This serves to prevent continued reaction which would reduce olefin yields. Following the exchangers, the product gas passes to a separation system. The gas is first cooled to essentially ambient conditions and condensable products removed. It is then compressed to a high pressure to enable separation of lighter products. Prior to separation, any CO.sub.2 formed must be removed from the cracked effluent in order to avoid plugging and contamination problems in the downstream light olefins separation system. This is typically done using caustic scrubbing and the spent caustic must be neutralized prior to discharge to the environment. The other carbon oxide, CO, passes through the separation process but must be removed from the net hydrogen stream if that stream is to be utilized in hydrotreating units. Both of these operations have considerable capital and operating costs associated with them. In the normal pyrolysis process, carbon oxides are a result of the steam reforming reactions between the dilution steam and the hydrocarbon.
Dependent upon the feedstock, the light olefin separation system can be very complex and require a number of distillation towers. Further, the lower the molecular weight of the gas mixture, the higher the pressure and/or the lower the temperature has to be to achieve separation. The hydrogen content of the gas mixture is a key component in defining the molecular weight. The compression for both the product gas and the refrigeration systems represents a major capital and energy cost for the process. The lower the molecular weight, the greater the compression energy required. Thus high hydrogen concentration in the product gas impacts the process in two ways, increasing compression costs due to molecular weight and increasing refrigeration costs due to lower temperature requirements.
The use of oxygen in dehydrogenation processes has been considered in the past. In a typical oxy-dehydrogenation process, rather large quantities of oxygen are used at relatively low temperatures. In these processes, feed and oxygen are passed over a catalyst and reaction occurs. The oxygen quantities are typically high; on the order of 50% by volume or more of the hydrocarbon feed. These quantities are required since the oxy-dehydrogenation reaction is the only reaction occurring at the relatively low temperatures (300.degree.-500.degree. C.) involved. Thus to achieve commercially viable conversions, high oxygen is required. However, at high oxygen concentration, the reaction selectivity is low and high amounts of CO and CO.sub.2 are formed over the mixed metallic oxide catalysts even at relatively low temperatures (300.degree.-500.degree. C.). Selectivity in this case is defined as the oxygen going to water compared to the oxygen resulting in carbon oxides. At these low temperatures, conversion is controlled by oxygen flow and in practice conversion must be limited to maintain high selectivity. These processes are thus generally uneconomic due to low conversion per pass (leading to high recycles), high levels of CO.sub.x formed, and the high cost of their removal. In addition, unlike the conventional process, relatively high concentration of co-product oxygenates are formed which are undesirable in the overall process. Thus in any pyrolysis operation that considers the use of oxygen, it is important that the CO.sub.x products be minimized. The oxygen should selectively react with the hydrogen to as high an extent as possible forming water as opposed to CO.sub.x. The oxidative dehydrogenation reaction is net exothermic compared to the endothermic pyrolysis reaction. Heat removal places an additional limitation on conversion for oxydehydrogenation since, as temperature increases with the higher levels of oxygen required for conversion, selectivity decreases substantially.
The use of smaller quantities of oxygen at elevated temperatures (where pyrolysis reactions are significant) has been considered. See the article, "Oxidative Pyrolysis of Propane", Layokum, Stephan K., Ind Chem. Process Des. and Dev., vol 18, No. 2, 1979. In this case, small quantities of oxygen (2-3% of feed) were added to propane and reacted in an empty laboratory scale pyrolysis tube. Nitrogen was used as a diluent instead of steam. In that article, the authors report slight improvements in primary olefin selectivity (to propylene). Residence times were controlled at 0.1 seconds and reaction temperatures were 600.degree.-700.degree. C. No reference is made to CO.sub.x production or to the impact of such operation on ethylene furnaces and no catalyst was used.
Recently, there have been a number of literature articles that have addressed the use of oxygen for ethane pyrolysis. In all these cases, oxygen is added to the feedstock flow and passed through the reaction tube. No catalyst is involved. The authors discuss the reduction of the total fired energy of the system by utilizing the heat from the combustion reactions. In "Coupling of Thermal Cracking with Non-Catalytic Oxidative Conversion of Ethane to Ethylene", Choudary, V. R., et al., AlChE Journal, June 1997, vol 43, No. 6, the authors examine ethane pyrolysis with oxygen with limited oxygen (0 to 20%), no catalyst, and at low residence times and high temperatures (600.degree.-850.degree. C.). Their studies show substantial increases in CO.sub.x and substantial decreases in selectivity to ethylene as oxygen increases. Increasing oxygen from 0 to 10% by vol of feed increased selectivity to CO from 2% to 7%, selectivity to CO.sub.2 from 0 to 0.3%, and decreased selectivity to ethylene from approximately 75 to 60%. In "Oxidative Pyrolysis of Ethane", Qi Chen et al, Ind Eng. Chem. Res, 1997, 36,3248-3251, the authors also use an empty laboratory scale pyrolysis tube with no catalyst and show that the addition of 7 vol % oxygen to ethane at constant process conditions increases selectivity to CO from essentially 0 to 5%, CO.sub.2 selectivity increased to 0.3%, and ethylene selectivity decreased from 85 to 80%. The increased carbon oxide products formed in the non-catalytic processes have a definite negative effect on selectivity.